lldpcli: switch to GNU Readline.

libedit is not available on old versions of Linux distro. We try to
use Readline API instead which may be compatible with the libedit
version available in BSD.

This means that we have to write our own tokenizer. This also means
that we can make Readline optional.
lldpcli: make readline library optional.

And improve compatibility. We should be compatible with BSD
libedit. However, we are not compatible with older versions that lack
`rl_insert_text()` and `rl_delete_text()`.